1. Field of Invention
The present invention relates to implantable medical devices, such as endoprostheses. More specifically, the present invention is related to an apparatus and method for selectively coating such devices.
2. Description of Related Art
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumens or cavities such as, for example, those in urinary tracts and bile ducts. Stents are used in the treatment and amelioration of disorders that include, but are not limited to, tumors in organs such as bile ducts, esophagus, and trachea/bronchi; benign pancreatic disease; coronary artery disease; carotid artery disease; and peripheral arterial disease.
Peripheral arterial diseases include, but are not limited to, atherosclerosis, which includes fibrous lesions and vulnerable plaque lesions; and restenosis, where “restenosis” can be a post-treatment condition that includes, for example, the reoccurrence of a stenosis in a blood vessel or heart valve after it has been treated, for example, by balloon angioplasty or valvuloplasty with an otherwise apparent success. Vulnerable plaque is a type of fatty build-up in an artery thought to be caused by inflammation and can be covered by a thin fibrous cap that can rupture and lead to blood clot formation. The treatment of these and other conditions can benefit from a localized delivery of an agent. Stents may be used to reinforce vessels and prevent restenosis following angioplasty in the vascular system and to deliver drugs from a solid structure at the lesion site.
A treatment involving a stent includes both delivery and deployment of the stent. Delivery and deployment of a stent may be accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into the lumen, advancing the catheter in the lumen to a desired treatment location, expanding the stent at the treatment location, and then removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon, and the stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter using, for example, a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn to allow the stent to self-expand.
Stents are often modified today to provide drug delivery capabilities by coating them with a polymeric carrier impregnated with a drug or other therapeutic substance coated on a stent. A conventional method of coating includes applying a composition to a stent. The composition can include, for example, a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend. The composition can be applied, for example, by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving a coating containing the polymer and the therapeutic substance on the stent strut surfaces. The dipping or spraying of the composition onto the stent can result in coating all stent surfaces.
Some coating compositions need to be selectively applied to avoid problems that can occur during the manufacture or during the use of the medical device. For example, a polymeric coating on the inner surface of the stent can increase the coefficient of friction between the stent and the balloon of a catheter assembly on which the stent is crimped. Some polymers can also have a “sticky” or “tacky” consistency. If the polymeric material either increases the coefficient of friction or adheres to the catheter balloon, the effective release of the stent from the balloon after deflation can be compromised. The coating or parts thereof, for example, can be pulled off the stent during the deflation and withdrawal of the balloon that occurs following placement of the stent in a patient. Adhesive, polymeric stent coatings can also experience extensive balloon-sheer damage after deployment, which can result in a thrombogenic stent surface and possible embolic debris. Further, the stent coating can stretch when the balloon is expanded and result in delamination as a result of shear stress.
In general, having a coating on the luminal surface of a stent can detrimentally impact the stent's deliverability as well as the coating's mechanical integrity. Moreover, from a therapeutic standpoint, the therapeutic agents on the inner surface of the stent can be washed away by blood flow and provide for an insignificant therapeutic effect, in addition to being a wasteful application of the therapeutic agent. In contrast, the agents on the outer surface of the stent contact the lumen of an occluded vessel and provide for a more efficient delivery of the agent directly to the tissues. Reducing the amount of ineffective and potentially detrimental material, such as the residual luminal coating of a stent, is desirable with respect to stent coating techniques for at least the reasons stated above.
Accordingly, a skilled artisan would appreciate an improved method for selectively coating a medical device. An improved method of selectively coating only the abluminal surface of a stent can improve the biological outcome, flexibility of a stent, and coating design. Such a method would, for example, increase the flexibility of a coating process by allowing more freedom in designing a coating process and providing products with improved mechanical and therapeutic benefits. Moreover, a selective coating process design that can be retrofitted to existing coating processes would be appreciated and valued by those skilled in the art. In particular, creating a more robust spray coating process would be a great contribution to the art, since spray coating processes have already undergone a great deal of development and are used widely in the field.